So the low end of the FX700 is unnaturally boosted, emphasizing the fun factor, then?
What would you then consider to be the most natural/accurate, if I may use those terms interchangeably, sounding IEM you've heard?
Do IEMs of this sound signature typically have a flat frequency response, and are they commonly referred to as "boring" by others?
Yeah, I believe it's boosted past the needed compensation point. I would point out that it's still nothing like the hump on IEMs like the MDs or IE8s that are more problematic. I will also add that it's my favorite modestly bass boosted (6db+estimate) IEM by far.
I personally think there are many so called top-tier IEMs, that I wouldn't place that high top because of their slightly excess coloration (IE8, W3, etc). In general I think BAs get a lot closer to the source, they're overall less peaky and can sound more effortless. Most natural/accurate IEM will be hard to judge as there are many products out there that get close enough that picking one will then be based on experience/preference. I do think there is some guidelines involved though. I already mentioned compensation of the bass and treble, but James' forward projection guideline is also quite sound, though I personally place that far behind frequency response. Off the top of my head, the SE530/5, EX1000, GR07, TF10, EQ-5, EQ-7 and K2 (more unmentioned) will be among the most accurate IEMs I've heard. I do dislike some of the mentioned though, mainly because of a slightly harsh or forward treble (K2, EQ-5, EX1000?) despite nice bass-midrange balance. Each have their share of flaws, it's about what has a better synergy for you because despite aiming to be accurate, you're still coloring the sound. There are so many factors that pull you away from the source that some coloration is inevitable.
FXT90 if looking for more bass and mids, while retaining a somewhat similar approach to the TF10. Even then, they're quite different. FX700 won't have forward mids.